Peel and grate the onion (this releases the onions juices, which can be discarded). Finely chop the garlic. Heat the olive oil in a large saucepan over a medium heat, and add the onion and garlic, gently frying until the onion goes translucent.

Chop the cherry tomatoes in half, and add to the onion and garlic mixture. Continue to cook gently until the tomatoes are soft and releasing their juices.

Add the passata and water, and start to season to taste. Bring to the simmer and cook for a further 30 minutes, stirring occasionally. Add more water to achieve desired consistency, and season again after cooking if necessary.

Press the tomato mixture through a sieve, so you are left with a smooth tomato sauce. Store in an air tight container in the fridge for up to a week, or in the freezer for up to 3 months.